Press section of a paper machine in which an extended-nip press is used

Abstract: The invention concerns a press section of a paper machine, through which press section the paper web (W) is passed as a closed draw on support of a press fabric (11, 28, 36) and of a smooth roll face (31, 39'). The first nip in the press section is an extended-nip press (NP 1 ) through whose press zone two opposite press fabrics (11, 28) that receive water have been passed. The upper press fabric in the extended-nip press (NP 1 ) is a pick-up fabric (11), which carries the paper web (W) from the forming wire (10). In the press section, there are at least two other nips (N 1 , N 2 ; NP 2 ', NP 3 ') that have been formed in connection with the smooth-faced (31) centre roll (30). The centre roll (30) is fitted at a level higher (H 1 ) than the level of the extended-nip press (NP 1 ). In the first one of the other nips (N 1 ; NP 2 '), the press fabric consists of the pick-up fabric (11), and the second other nip (N 2 ; NP 3 ') has a press fabric (33) of its own that receives water. The upper press component of the extended-nip press (NP 1 ) is a suction roll (13b) provided with a mantle (13'') with through perforations. After the extended-nip press (NP1), the running direction of the paper web (W) has been turned at an angle a, which has been chosen as a ≥ ∼45 0 .

Description: The invention concerns a press section of a paper machine, which press section comprises at least three consecutive nips and wherein the paper web to be dewatered is passed as a substantially closed draw on support of a press fabric and of a smooth roll face or of a corresponding transfer band. It is known from prior art, in the press sections following after the former section of a paper machine, to use one or several extended-nip presses, whose press zone in the running direction of the web is substantially longer than in roll press nips. Earlier, extended-nip presses were used more commonly in board machines and with thicker paper grades only, but extended-nip presses are also being introduced with thinner paper grades, such as newsprint and fine papers. With respect to the prior art most closely related to the present invention, reference is made to the following documents : FI-B-91898 (equivalent to DE-A-3 808 293), DE-U-8 805 966, DE-A-4 026 021, FI-B-75382, FI-B-72195, US-A-4 257 844, US-A-4 704 192 and US-A-5 120 399. Further, reference is made to the paper in the journal Wochenblatt für Papierfabrication 19 (1993), pages 180...182 "Die Flexonip®-Pressen" , which paper describes some of the latest extended-nip press constructions of Messrs. J.M. Voith GmbH. The prior art press sections, including those described in the documents mentioned above, have involved certain problems and need of development, which have contributed to justifying the present invention. The most important ones of these problems and drawbacks will be dealt with in the following. In the press sections described in document FI-B-91898 (Figs. 3 and 4), a drawback is the transfer of the web from one extended-nip press into the second and/or third extended-nip press on the face of the glide-belt mantle of the extended-nip press, because that requires a smooth face which does not receive water and which has good web transfer and adhesion properties. In this document, the first nip is not an extended nip that removes water efficiently in both directions, and the overall concept is not a compact press section of several nips. In document DE-U- 88 05 966, the drawbacks include, above all in the embodiments shown in Figs. 1 and 2, the press section consisting of two separate extended-nip presses alone, in which press section the high-load pressing in both of the extended nips is carried out on the face of the same press felt and, moreover, the web is transferred over the long distance between the nips between two re-wetting felts and on the face of one felt. The geometries of the press sections as shown in Figs. 3, 5 and 6 in said document are more compact, but the same press felt runs through both of the extended nips, and the water-absorbing capacity of the felt is no longer sufficiently efficient in the second extended nip. The transfer of the web from one extended nip into the other relies fully on the differences in the surface structures of the press felts, and no devices have been used to secure the web transfer. The press sections illustrated in Figs. 1,1a,2,3,3a and 3b in document DE-A-4 026 021 involve the drawback that transfer means are used which run through all the press nips and which have a dewatering capacity lower than that of a felt. Since the largest amount of water is removed in the first nip, the dewatering in one direction is limited by the use of said transfer means. The transfer means that are used in the constructions as shown in Figs. 2,2a,3,3a,3b consist of the glide belt of the extended-nip press, in which case the dewatering in the first nip in one of the directions is excluded completely. Said glide belt must be fully impervious and smooth. In document FI-B-72159, in document FI-B- 75,382 or in document US-A- 4,257,844, the use of an extended-nip press with a high dewatering capacity is not suggested as the first nip, or elsewhere, nor is a closed web transfer to the dryer section suggested. In document US-A- 5,120,399, as the first nip, just the use of a single-felt extended nip that removes water in one direction is suggested, and the press sections comprise two nips, and no compact multi-nip press section is suggested. A press section comprising the features of the pre-characterizing clause of claim 1 is known from document EP-A-0 289 477, in particular its Figure 10. In the known press section, the first nip is formed as a roll nip between a press roll and a suction roll. The press section is designed such that the dry solids content after the press section is comparatively high, and it comprises a hot-pressing device which is arranged downstream of the third nip. The hot-pressing device comprises a heated cylinder and a press shoe device which, in combination, form an extended nip. It is an object of the present invention to further develop the known press section such that a supported and reliable transfer of the web from the former section to the press section and through the entire press section is achieved even at high web speeds, while ensuring a high dewatering capacity and sufficiently symmetric dewatering. Moreover, it is an object of the present invention to provide a compact press section construction, in particular in the machine direction. This is an important objective in particular in such modernizations of paper machines in which it is necessary to replace an existing press section comprising roll nips (e.g., the Patentee's Sym-press II® ) by a new press section which has a higher dewatering capacity. It is a particular object of the present invention to provide a supported and reliable transfer of the web through the entire press section even at high web speeds, e.g. web speeds in the order of 25....35m/s. These objects are achieved by the paper machine according to claim 1. In the paper machine according to the invention, the first nip in the press section is an extended nip of a first extended-nip press. The essence of the invention consists in the combination of the following features: the first nip in the press section is an extended-nip of a first extended-nip press through whose press zone two opposite press fabrics that receive water are passed, so that in the first extended-nip press the dewatering takes place in two directions through both faces of the paper web; the upper press fabric in said first extended-nip press is a pick-up fabric, which carries the paper web from the forming wire on its lower face; at least two other nips in the press section are formed in connection with a smooth-faced centre roll, which centre roll is fitted at a level substantially higher than the level of the extended-nip press, and of which other nips, in the first other nip the press fabric consists of said pick-up fabric, and the second other nip has a press fabric of its own that receives water, the first extended-nip press comprises as an upper press component a suction roll provided with a mantle with through perforations, and after said first extended-nip press, the running direction of the paper web is turned at an angle a, which has been chosen as a ≥ ∼45°. According to the invention, in the first extended-nip press in the press section, efficient two-sided dewatering is achieved, which contributes to securing the symmetry of the structure of the web in the z-direction and the symmetry of the density and porosity of the web faces. Also, in the first extended-nip press, a relatively high dry solids content is obtained for the web, so that the direction of the web can be changed substantially after the extended-nip press without a risk of breaks, which contributes to the possibility to provide a press section which has a compact construction and which also occupies a sufficiently small space in the machine direction in view of modernizations. Moreover, in the present invention, through the whole press section, a closed or substantially closed draw of the web is accomplished, with just very short free draws. In particular with thicker paper grades and/or with web speeds higher than average, an extended-nip press is used as the last nip in the press section, in which extended-nip press the draining direction is contrary to the direction in the roll nips preceding the extended-nip press, which contributes to securing or improving the symmetry of the web in the z-direction. In the following, the invention will be described in detail with reference to an exemplifying embodiment of the invention illustrated in the only figure in the accompanying drawing, the invention being by no means strictly confined to the details of the embodiment. The only figure is a schematic side view of an exemplifying embodiment of the invention. According to the shown embodiment, the paper web W is separated from the forming wire 10 and transferred at the pick-up point P onto the pick-up felt 11, being aided by the suction zone 12a of the pick-up suction roll 12. The pick-up felt 11 operates as a water-receiving upper fabric in the first extended nip NP 1 , into which the web W is transferred on the lower face of the felt 11. In the extended nip NP 1 there is a water-receiving lower felt 28, which is guided by guide rolls 29. The lower press component in the first extended nip NP 1 is a hose roll 20 provided with a flexible mantle 21. In the interior of the mantle 21, there is a press shoe 22, which is loaded by hydraulic cylinders, whose pressures can be regulated in order to regulate the level and the distribution of the compression pressure in the extended nip NP 1 both in the direction of progress of the web W and in the transverse direction. The hose roll 20 is a press component in itself known, and in respect of its construction, reference is made by way of example to document FI-B-96789, Figs. 10;11;12. The upper press component of the extended nip NP 1 is a suction roll 13b which is provided with a perforated mantle 13" and in the interior of whose mantle there are two successive suction zones 13c and 13d. Of said zones, the first one 13c is placed facing the press zone of the extended nip NP 1 , and the next zone 13d in the sector on which the run of the web W and of the upper felt 11 is turned to the vertical direction. The suction roll 13b may be used, e.g., with thinner paper grades when the loading in the extended nip NP 1 is lower than average, in which case the mantle 13" can be made to withstand the pressure load in the extended-nip press. In the first extended-nip press forming the extended nip NP 1 , even at high web running speeds, a sufficiently long dwell time and efficient dewatering are obtained for the web, said dewatering being additionally two-sided and symmetric. The dry solids content of the web W on its arrival in the extended nip NP 1 is typically in a range of 12...20 %, and immediately after the extended nip NP 1 the dry solids content of the web W is, as a rule, in a range of 30...40 %. The proportion of the efficient dewatering taking place in the extended nip out of the entire dewatering taking place in the press section is typically in a range of 30...75 %, most commonly in a range of 35...55 %. The upper roll 13b in the extended nip NP 1 turns the running direction of the upper felt 11 and of the web W, which is transferred on support of said felt, from substantially horizontal to substantially vertical or to relatively steeply upwards inclined. This change (angle a) in the running direction of the upper felt 11 and of the web W contributes to an improved utilization of the space taken by the press section. As a rule, said angle a ≥ ∼45°. By means of a steam box 14b or a corresponding infrared radiator operates, the free face of the web W is subjected to a heating radiation or medium, whereby the removal of water is promoted in the following nips N 1 ,N 2 ,NP 2 by making use of modes of effect in themselves known. Said relatively large change in the direction even at high web speeds, e.g. in the speed range of 25...35 m/s, is permitted by the two-sided efficient dewatering carried out in the first extended nip NP 1 , because of which dewatering the dry solids content and the strength of the web W have been raised to a level that permits even a large change in direction a and high centrifugal forces. The press section includes a smooth-faced 31 centre roll 30, in whose connection there are preferably two roll nips N 1 and N 2 . The length of the press zones in the roll nips N 1 and N 2 is substantially, typically by almost one order, shorter than in the extended nips NP 1 and NP 2 , in which the length of the press zone is typically 100...300 mm. Said roll nips N 1 ,N 2 are placed on successive upper quarters of the smooth-faced 31 centre roll 30, by means of which arrangement a favourable distribution of the nip loads on the centre roll 31 is obtained. The upper felt 11 of the extended nip NP 1 , which is also the pick-up felt, additionally operates as a water-receiving press fabric in the first roll nip N 1 , after which the web W follows the smooth face 31 of the centre roll 30, being separated from the upper felt 11. The second roll nip N 2 is formed by the centre roll 30 together with the hollow-faced 32' press roll 32. Through the second roll nip N 2 , the press felt 33 runs, which receives water and is guided by the guide rolls 34. The web W is separated as a short free draw from the smooth face 31 of the centre roll 30, being transferred onto the lower felt 36 as guided by the guide roll 35 and aided by a suction zone of the transfer-suction roll 37, which lower felt 36 operates as the lower press fabric in the second extended nip NP 2 . The second extended nip NP 2 is formed by a lower hose roll 40, which is provided with a flexible mantle 41 and a loading shoe 42. The upper roll of the extended nip NP 2 is a solid-mantled, preferably variable-crown and smooth-faced 39' press roll 39. After the extended nip NP 2 , the web W follows the smooth face 39' of the press roll 39, from which it is separated as a short free draw W 1 , being transferred over the paper guide roll 53 onto the drying wire 52. The press roll 39 and/or the centre roll 30 may be heated, and the heating can be carried out, e.g., by means of hot water that circulates through bores in the roll mantle, in respect of which details reference is made to document EP-A1-597814. Before the second extended nip NP 2 , inside the loop of the lower felt 36, there is a suction box 38 or equivalent. The latter guide roll of the lower felt 36 is denoted with the reference numeral 54. On the upper sector of the upper backup roll of the second extended nip NP 2 , there is a doctor 43, by whose means the paper web passing to broke is transferred into a transverse broke trough or onto a transverse broke conveyor 44 to be passed to the side of the paper machine and further into the pulper. In some applications, the extended nip NP 2 can be replaced by a corresponding roll nip, which is represented by the reference denotation N 3 in parentheses. In such a case, the upper roll 39 is a preferably smooth-faced press roll, and in the position of the lower hose roll 40 there is a hollow-faced and solid-mantled rigid press roll, if necessary, a variable-crown press roll. The suction roll 13b considerably facilitates the transfer of the web W into the roll nip N 2 as the suction zones 13c and 13d of the suction roll 13b transfer the web W reliably. Above, a preferred embodiment of the invention has been described in which, in connection with the centre roll 30, there are expressly two roll nips N 1 and N 2 . In some cases, in particular with thicker paper grades or with board, the invention can also be carried into effect so that, in the position(s) of one or both of the roll nips N 1 and/or N 2 , an extended nip/nips is/are employed, which is/are illustrated in the figure by the dashed lines and denoted with the references NP 2 ' and NP 3 ' in parentheses. In such a case, the hose roll 70 forms the second extended nip NP 2 ', and the hose roll 80 forms a possible third extended nip NP 3 ', if any, in connection with the centre roll 30. If an extended nip NP 2 ' and/or NP 3 ' is/are employed in connection with the centre roll, as a rule, such an embodiment is preferable in which the first nip in connection with the centre roll is a roll nip N 1 and the second nip is an extended nip NP 3 ', but other combinations of nips are also possible. The press section shown in the figure is best suitable for printing paper grades, such as newsprint, SC-paper, LWC base paper, and fine paper, but said press sections can also be used for boards, such as linerboard or corrugating medium. In the following, the widest and the preferable ranges of variation of the linear loads in the various nips in a press section in accordance with the present invention will be given, without, yet, restricting the invention to these values. First extended nip NP 1 = 100...1400 kN/m, preferably 200...800 kN/m Second extended nip NP 2 = 150...1400 kN/m, preferably 400...1200 kN/m Second nip (1st roll nip) N 1 = 50...150 kN/m, preferably 70...130 kN/m Third nip (2nd roll nip) N 2 = 70...200 kN/m, preferably 90...150 kN/m Fourth nip (3rd roll nip) N 3 = 70...200 kN/m, preferably 90...150 kN/m The frames of the press section are normal frames that permit quick replacements of felts and rolls. In the extended nips NP 1 ;NP 2 (NP 2 ',NP 3 '), it is preferable to use press felts slightly heavier and thicker than normal, because the amount of water that is removed in the extended nip NP 1 ,NP 2 (NP 2 ',NP 3 ') is larger and the high press impulse tends to produce a marking of the fabric or of the hollow face on the paper. In an extended-nip press, the glide-belt mantle 21;41;81 is preferably hollow-faced, such as grooved, blind-drilled, or provided with other recesses. According to the invention, a particularly compact press section is produced so that, for example, in modernizations of paper machines, in which the dewatering capacity of the press section is increased, e.g., in order to increase the running speed of the paper machine, the press section can be accommodated in the place of an existing press section, e.g., consisting of three or four nips which are exclusively roll nips, such as in place of the applicant's Sym-Press II® press. The various details of the invention may show variation within the scope of the inventive idea defined in the claims and differ from what has been stated above by way of example only.

Claims: Press section of a paper machine for dewatering a paper web (W) formed on a forming wire (10) of a former section, wherein the press section comprises at least three consecutive nips (NP 1 , N 1 , N 2 , NP 2 ', NP 3 ') and wherein the paper web (W) to be dewatered is passed as a substantially closed draw on support of a press fabric (11, 28, 36) and of a smooth roll face (31, 39') through the press section, wherein the first nip (NP 1 ) in the press section is formed by a press which comprises as an upper press component a suction roll (13b) provided with a mantle (13") with through perforations, wherein two opposite press fabrics (11, 28) that receive water are passed through the press zone of the first nip (NP 1 ) with the web being disposed between said two press fabrics, so that in the first nip (NP 1 ) the dewatering takes place in two directions through both faces of the paper web (W), wherein the second nip (N 1 , NP 2 ') and the third nip (N 2 , NP 3 ') in the press section are formed in connection with a smooth-faced centre roll (30) which is fitted at a level substantially higher than the level of said first nip (NP 1 ), wherein the upper press fabric in said first nip (NP 1 ) is a pick-up fabric (11) for picking up the paper web from the forming wire (10), said pick-up fabric (11) carrying the paper web on its lower face through said first nip (NP 1 ) and through said second nip (N 1 , NP 2 '), wherein, after said first nip (NP 1 ), the paper web (W) is separated from the lower press fabric (28) by means of said suction roll (13b), and the running direction of the upper press fabric (11) and of the paper web (W) is turned at an angle (a) which is equal to or greater than approximately 45°, and. wherein said third nip (N 2 , NP 3 ') has a press fabric (33) of its own that receives water, characterized in that said first nip is an extended nip (NP 1 ) of a first extended-nip press. Press section as claimed in claim 1, characterized in that said suction roll (13b) comprises two successive suction zones (13c, 13d), of which the first zone (13c) is placed in the area of the press zone of the first extended nip press and the other zone (13d) in the turning sector where the upper fabric (11) and the paper web (W) turn their direction at said angle (a). Press section as claimed in claim 1 or 2, characterized in that at least one of said second and third nips is a roll nip (N 1 and/or N 2 ). Press section as claimed in claim 1 or 2, characterized in that at least one of said second and third nips is an extended nip (NP 2 ' and/or NP 3 '). Press section as claimed in any of claims 1 to 4, characterized in that, said second nip (N 1 , NP 2 ') and said third nip (N 2 , NP 3 ') are arranged at opposite upper quarters of the center roll (30). Press section as claimed in any of the claims 1 to 5, characterized in that said first extended-nip press comprises a lower press component which is formed by a hose roll (20) provided with a flexible mantle (21) against whose inner face a press-glide shoe (22) loaded by means of the pressures of a pressure medium acts in the extended-nip press zone. Press section as claimed in any of the claims 1 to 6, characterized in that downstream of said third nip, a separate fourth press nip (N 3 , NP 2 ) is fitted in the press section. Press section as claimed in claim 7, characterized in that said fourth nip (NP 2 ) is formed by a separate second extended-nip press which is provided with one lower press fabric (36) and in which a lower press component is a hose roll (40) provided with a flexible hose mantle (41) and an upper press component is a smooth-faced press roll (39) on whose smooth face (39') the paper web (W) is passed onto a drying wire (52) in a dryer section or to its proximity. Press section as claimed in any of the claims 1 to 8, characterized in that, after the first extended-nip press, the free face of the paper web (W) is subjected to a medium that heats the web and is fed from a steam box preferably in an area in which a suction zone (13d) is placed at the opposite side of the paper web (W), or is subjected to an infrared radiator device to intensify the dewatering in the second and consecutive press nips. Press section as claimed in any of the claims 1 to 9, characterized in that the center roll (30) of the press section and/or the press roll (39) of the second extended-nip press is/are heatable.

Inventors: LAAPOTTI JORMA, LAAPOTTI, JORMA
Applicants: METSO PAPER INC, METSO PAPER, INC.
Application year: 1994
Main classification: D21F3
IPC codes: D21F3